The present invention pertains to an apparatus for driving the needle of a sewing machine of the type having a needle clamp mounted for sliding movement on a needle guide bar that is fixedly supported by the casing or housing of the machine, and which is pivotably connected to one end of a control lever that is pivotably mounted intermediate its ends on a crank forming one end of an oscillatably driven shaft of the machine. The opposite end of the control lever is operatively connected to a pivotable lever element which is pivotably attached to the housing of the sewing machine. Needle driving apparatuses of this type are well known to those conversant in the art and provide a structure which produces a relatively light load on the supporting bearings and the pivoting elements so that their use is considered especially advantageous in sewing machines operating at high speed.
These known driving apparatuses are distinguished by the fact that the needle clamp slides easily and without interference along the needle bar which is fixed and straight, even though the path travelled by said needleclamp deviates slightly from a straight line movement. In practice the needle clamp travels a path which resembles an elongated letter "S" that is substantially lengthened in an upwardly direction and with the width thereof being approximately 1 hundredth of a millimeter.
Due to the configuration of the needle path, the needle clamp is caused to deviate a few microns in value, relative to the straight path which the needle bar represents. It has been established that this deviation is considerably greater relative to the lower end of the needle path so that the portion of it utilized during the formation of stitches by the needle clamp is in the area adjacent said lower end and whereby its length depends on the dimensional characteristics of the driving apparatus to which the said needle clamp is connected.
During the formation of each stitch, the needle carried by the needle clamp passes through the material to be sewn to a position below the machine's needle plate where in a known matter it cooperates with the machine's loopers so as to effect proper linking of the threads carried by them. To consistently accomplish this, the location at which the needle cooperates with the loopers must always occur at the same level.
Additionally, during the formation of stitches, the needle is raised a sufficient distance to clear the workpiece beneath it so that no interference will be had with projections, crosswise stitching, pleats, etc., which could prevent it running freely.
When sewing very thick materials, the needle's upward travel must be adequately increased to maintain the same working conditions for medium thick or thin materials, and the location at which the needle cooperates with the loopers beneath the needle plate must remain unchanged.
In the known art, the means of adjusting the needle's upward travel relative to differences in thickness of workpiece materials is accomplished by utilizing the movable pivoting element of the above-mentioned control lever in a different location from that for which it was originally intended for the same type of driving apparatus. This obviously means that no change is made in the general dimensions of the above type of driving apparatus. Originally, the movable pivoting element was formed by a pivoting lever of pre-determined length, which was pivoted at its lower end to the end of the control lever most remote from the needle and pivoted at its upper end on the machine housing and so disposed as to be substantially parallel to the needle bar when in its median position. This arrangement of the pivoting lever made it impossible to increase the needle's upward travel by any appreciable amount without reaching the critical area of the path travelled by the needle clamp. In view of this, a mirror inversion of the above pivoting lever was made and mounted to operate with the control lever. Although this arrangement accomplished its intended purpose, it created an imbalance in the driving apparatus for this combination of levers could not perform their intended functions with the smoothness with which the original form of pivoting lever provided.
As is well known, sewing accessories such as chain stitching equipment, separating knives, additional conveyors, etc., are frequently provided in the area of the work surface of the machine rearwardly of the needle, and must be disposed so as not to cause an interference with the workpiece during the seaming operation. This results in a limited amount of area available for the pivoting lever which requires that it be displaced from the plane in which the control lever usually functions. The levers arranged in this manner initiates wear between the coupling pin and the associated seatings of said levers, as well as between the elements forming the fulcrum in the housing for the pivoting lever, and thus prevent the sewing machine from performing as efficiently as is possible. An object of the present invention is that of overcoming the limitations and disadvantages with the known driving apparatuses while maintaining substantially the same dimensional characteristics. The technical problem to be resolved in accomplishing this object is that of subdividing all the possible thicknesses of materials to be sewn into two distinct groups and predetermining the required amount of travel for the needle clamp that is appropriate for each group and which will function by means of a single driving apparatus having a pivoting lever which can be mounted on the machine's housing in a manner whereby it can be selectively positioned in either of two distinct locations each of which is adapted to provide a different length of travel for the needle clamp.